Transcript Part 2
Chapter 2 Communications Systems Modeling Postacademic Interuniversity Course in Information Technology – Module C1 p1 Contents • • • • • Introduction Example: a diplomatic negotiation Principles of a layered model Connection oriented and connectionless communications Communication networks according to a layered model Postacademic Interuniversity Course in Information Technology – Module C1 p2 Contents • • • • • Introduction Example: a diplomatic negotiation Principles of a layered model Connection oriented and connectionless communications Communication networks according to a layered model Postacademic Interuniversity Course in Information Technology – Module C1 p3 Layered Network Models • The problem: Complexity of networked systems • The solution: Decomposition into independent subsystems • Layered network model – Introduced by IBM (SNA, 1971) – Standardized by ITU (X200) and ISO (7498) – Method universally adopted – Standards not really obeyed in practice Postacademic Interuniversity Course in Information Technology – Module C1 p4 Contents • • • • • Introduction Example: a diplomatic negotiation Principles of a layered model Connection oriented and connectionless communications Communication networks according to a layered model Postacademic Interuniversity Course in Information Technology – Module C1 p5 A diplomatic negotiation Political arguments Service Access Points Translation Cryptography Transmission Postacademic Interuniversity Course in Information Technology – Module C1 p6 A diplomatic negotiation Political arguments Service Access Points Single language messages Cryptography Transmission Postacademic Interuniversity Course in Information Technology – Module C1 p7 A diplomatic negotiation Political arguments Service Access Points Single language messages Encrypted messages Transmission Postacademic Interuniversity Course in Information Technology – Module C1 p8 A diplomatic negotiation Political arguments Service Access Points Single language messages Encrypted messages Electromagnetic waves Postacademic Interuniversity Course in Information Technology – Module C1 p9 Contents • • • • • Introduction Example: a diplomatic negotiation Principles of a layered model Connection oriented and connectionless communications Communication networks according to a layered model Postacademic Interuniversity Course in Information Technology – Module C1 p10 Layered Model Definitions Level (n+1) entity Virtual level n+1 communication path using a level n+1 protocol Level (n+1) entity Physical Communication Path Level n Service Access Points Level (n) entity Virtual level n communication path using a level n protocol Level (n) entity Level n Addresses Level (n-1) Service Provider Postacademic Interuniversity Course in Information Technology – Module C1 p11 Layered Model Definitions Example: The Telephone Service Virtual Interpersonal communication path Physical Communication Path Telephone Service Access Points Telephone Numbers Telephone Network Postacademic Interuniversity Course in Information Technology – Module C1 p12 Layered Model Definitions Protocol Data Units Level (n+1) entity (n+1)PDU Level (n+1) entity PDU encapsulation Level (n) entity (n)PDU Level (n) entity Level (n-1) Service Provider Postacademic Interuniversity Course in Information Technology – Module C1 p13 Contents • • • • • Introduction Example: a diplomatic negotiation Principles of a layered model Connection oriented and connectionless communications Communication networks according to a layered model Postacademic Interuniversity Course in Information Technology – Module C1 p14 Connection Oriented Communications Typical example: A telephone conversation 3 step process: 1. Establish connection using the signaling protocol 2. Use connection using an informal protocol being billed for duration of usage 3. Terminate connection using the signaling protocol Postacademic Interuniversity Course in Information Technology – Module C1 p15 Connection Oriented Communications A temporary communication channel is built through the telephone network Postacademic Interuniversity Course in Information Technology – Module C1 p16 Connection Oriented Communications • Network has a state : the list of active connections • Network can guarantee Quality of Service – Reservation of transmission capacity at connection establishment – Monitoring of data transfer and correction of transmission errors by retransmission • Network is fragile : state can be lost. • Application domain : – Very efficient for transferring high volumes of data – Mandatory if stringent Quality of Service requirements – Poor initial response times due to connection setup – Inapplicable if nodes have high failure rate. Postacademic Interuniversity Course in Information Technology – Module C1 p17 Connectionless Communications Typical example: The postal service Letters, with an address and a stamp, are individually carried from a Post office box to a personal mailbox Postacademic Interuniversity Course in Information Technology – Module C1 p18 Connectionless Communications No connections are needed to carry mail Postacademic Interuniversity Course in Information Technology – Module C1 p19 Connectionless Communications • The network is stateless. • No guaranteed Quality of Service. – No reservation of transmission capacity – No error correction by retransmission • Network is robust : – No state stored in any node – Each node operates almost independently from others • Application domain : – Whenever single message response times are critical – Whenever nodes are unreliable Postacademic Interuniversity Course in Information Technology – Module C1 p20 Connection Policy Layer independance Level (n+1) entity Connectionless protocol Level (n+1) entity Connection oriented protocol Level (n) entity Level (n) entity Level (n-1) Service Provider Postacademic Interuniversity Course in Information Technology – Module C1 p21 Contents • • • • • Introduction Example: a diplomatic negotiation Principles of a layered model Connection oriented and connectionless communications Communication networks according to a layered model Postacademic Interuniversity Course in Information Technology – Module C1 p22 A two layers model. Networks Layer Postacademic Interuniversity Course in Information Technology – Module C1 Interoperability Connectivity Applications Layer p23 A three layers model. Networks Layer Postacademic Interuniversity Course in Information Technology – Module C1 Connectivity Internet & Transport Layer Interoperability Applications Layer p24 Internet & Transport Layers Applications Layer Transport Layer Internet Layer Networks Layer Postacademic Interuniversity Course in Information Technology – Module C1 p25 Internet & Transport Layers Example WWW Transport Layer TCP TCP Internet Layer IP SKYNET IP UUNET Networks Layer Postacademic Interuniversity Course in Information Technology – Module C1 p26 Introduced concepts • Layered communications system – protocol = horizontal convention – Service access point & address • Connection Oriented vs. Connectionless Communications • Applications and networks layers • Connectivity : – service offered by lower layers • Interoperability : – full communications services • Internet & Transport layers Postacademic Interuniversity Course in Information Technology – Module C1 p27 Bibliography To know More about network modeling Fred Halsall Data Communications, Computer Networks and Open Systems Fourth Edition Addison-Wesley 1995. ISBN 0-201-42293-X Recommended for this chapter Postacademic Interuniversity Course in Information Technology – Module C1 p28